NRK cells treated with picolinic acid are reversibly arrested in G1 and are very responsive to agents which elevate cyclic AMP levels. Other pyridine derivatives are ineffective. NAD levels are not changed, but nicotinamide partially overcomes the growth inhibition. The growth of transformed cells is also reversibly arrested but the block point is dependent upon the nature of the transforming agent. The results point to a role of NAD ion in growth control. BIBLIOGRAPHIC REFERENCES: Johnson, G.S., and Fernandez-Pol, J.A.: NRK Cells Synchronized in G1 by Picolinic Acid are Super-Sensitive to Prostaglandin E1 Stimulation. FEBS Letters 74: 201-204, 1977. Fernandez-Pol, J.A., Bono, V.A., and Johnson, G.S.: Control of Growth by Picolinic Acid: Differential Response of Normal and Transformed Cells. Proc. Natl. Acad. Sci. USA, in press, 1977.